This invention relates generally to a steam generating system having a coal or oil fired boiler and a regenerative air preheater. More particularly, the present invention relates to a steam generating system having a boiler and a rotary regenerative air preheater.
During the combustion process in the boiler, the sulfur in the fuel is oxidized to SO2. After the combustion process, some amount of SO2 is further oxidized to SO3, with typical amounts on the order of 1 to 2% going to SO3. The presence of iron oxide, vanadium and other metals at the proper temperature range produces this oxidation. Selective catalytic reduction (SCR) is also widely known to oxidize a portion of the SO2 in the flue gas to SO3. The catalyst formulation (primarily the amount of vanadium in catalyst) impacts the amount of oxidation, with rates ranging from 0.5% to over 1.5%. Most typical is around 1%. Therefore plants firing a high sulfur coal with a new SCR can see a large increase in the SO3 emissions, which produce a visible plume, local acidic ground level problems and other environmental issues.
Regenerative air preheaters condense or trap a portion of the SO3 in the flue gas. The SO3 is condensed as sulfuric acid at temperatures typically below 300xc2x0 F. Cold end acidic fouling of regenerative air preheaters creates a gradual increase in pressure drop. Sootblowing is generally utilized to reduce the rate of pressure drop build-up, but after some period of operation the air preheater must be cleaned by water washing. This is most typically accomplished by having an outage and shutting down the boiler. The maximum amount of pressure drop increase which is acceptable depends on the limitations of the existing fans, either the forced draft (air side), or induced draft (gas side) fans. The maximum acceptable pressure drop across the air preheater imposes limits on the design of the air preheater, principally limiting the number and type of heat exchange elements, thereby limiting the thermal efficiency of the air preheater.
Briefly stated, the invention in a preferred form is a method for increasing the efficiency of a steam generator system including a boiler producing a flow of flue gas containing SO3. An air preheater includes an air inlet and a flue gas outlet defining a cold end and a flue gas inlet and an air outlet defining a hot end. The flow of flue gas is received by the flue gas inlet, carried through heat exchange element basket assemblies, and discharged from the flue gas outlet, such that the flow of flue gas creates a pressure drop across the air preheater. A portion of the SO3 carried in the flue gas forms an acid which accumulates in the cold end of the air preheater, with the rate of acid accumulation depending on the amount of SO3 carried in the flue gas. The accumulating acid causes the pressure drop across the air preheater to increase from a maximum allowable clean condition pressure drop to a maximum allowable dirty condition pressure drop over the operating cycle of the steam generator system. The method comprises the steps of determining a reduced rate of acid accumulation which may be achieved by injecting an SO3 neutralizing or SO3 reactant additive material into the flue gas. A new maximum allowable clean condition pressure drop is calculated based on the reduced rate of acid accumulation. Modified heat exchange element baskets are created. The modified baskets have an increased heat transfer efficiency, compared to the conventional heat exchange element basket assembly, and a maximum allowable clean condition pressure drop substantially equal to the calculated new maximum allowable clean condition pressure drop. The conventional heat exchange element basket assemblies are replaced with modified heat exchange element basket assemblies. When the boiler is operating, the additive material is added to the flue gas.
Creating a modified heat exchange element basket includes identifying how the conventional heat exchange element basket assemblies may be modified to increase the heat transfer surface area and heat transfer. The cost of effecting each identified modification is determined. Finally, it is determined which of the identified modifications will most cost effectively produce the new maximum allowable clean condition pressure drop to provide the increased efficiency desired.
The steam generator system also generally includes fans for pushing and pulling the flue gas through the boiler. The maximum output of the limiting fan determines the maximum allowable dirty condition pressure drop (xcex94Pmax). The new maximum allowable clean condition pressure drop may be determined by calculating the increase in the pressure drop over the operating cycle attributable to the reduced rate of acid accumulation and subtracting the increase in the pressure drop over the operating cycle from the maximum allowable dirty condition pressure drop. Alternatively, the new maximum allowable clean condition pressure drop may be determined by calculating the increase percent decrease in acid accumulation over the operating cycle attributable to the reduced rate of acid accumulation (% xcex94P) and determining the maximum allowable dirty condition pressure drop with the formula xcex94Pmax/(1+% xcex94P).
It is an object of the invention to provide a cost effective steam generating system in which a large percentage of SO3 emitted by the boiler is removed in the installed regenerative air preheater.
It is also an object of the invention to provide a steam generating system in which fouling and corrosion problems associated with SO3 removal are minimized.